1. Technical Field
This invention relates to gas turbine engines in general, and to cooling passages disposed within a wall inside of a gas turbine engine.
2. Background Information
A typical gas turbine engine includes a fan, compressor, combustor, and turbine disposed along a common longitudinal axis. The fan and compressor sections work the air drawn into the engine, increasing the pressure and temperature of the air. Fuel is added to the worked air and the mixture is burned within the combustor. The combustion products and any unburned air, hereinafter collectively referred to as core gas, subsequently powers the turbine and exits the engine producing thrust. The turbine comprises a plurality of stages each having a rotor assembly and a stationary vane assembly. The core gas passing through the turbine causes the turbine rotors to rotate, thereby enabling the rotors to do work elsewhere in the engine. The stationary vane assemblies located forward and/or aft of the rotor assemblies guide the core gas flow entering and/or exiting the rotor assemblies. Liners, which include blade outer air seals, maintain the core gas within the core gas path that extends through the engine.
The extremely high temperature of the core gas flow passing through the combustor, turbine, and nozzle necessitates cooling in those sections. Combustor and turbine components are cooled by air bled off a compressor stage at a temperature lower and a pressure greater than that of the local core gas. The nozzle (and augmentor in some applications) is sometimes cooled using air bled off of the fan rather than off of a compressor stage. There is a trade-off using compressor (or fan) worked air for cooling purposes. On the one hand, the lower temperature of the bled compressor air provides beneficial cooling that increases the durability of the engine. On the other hand, air bled off of the compressor does not do as much work as it might otherwise within the core gas path and consequently decreases the efficiency of the engine. This is particularly true when excessive bled air is used for cooling purposes because of ineffective cooling.
One cause of ineffective cooling can be found in poor film characteristics in those applications utilizing a cooling air film to cool a wall. In many cases, it is desirable to establish film cooling along a wall surface. A film of cooling air traveling along the surface of the wall increases the uniformity of the cooling and insulates the wall from the passing hot core gas. A person of skill in the art will recognize, however, that film cooling is difficult to establish and maintain in the turbulent environment of a gas turbine. In most cases, air for film cooling is bled out of cooling apertures extending through the wall. The term xe2x80x9cbledxe2x80x9d reflects the small difference in pressure motivating the cooling air out of the internal cavity of the airfoil. One of the problems associated with using apertures to establish a cooling air film is the film""s sensitivity to pressure difference across the apertures. Too great a pressure difference across an aperture will cause the air to jet out into the passing core gas rather than aid in the formation of a film of cooling air. Too small a pressure difference will result in negligible cooling airflow through the aperture, or worse, an in-flow of hot core gas. Both cases adversely affect film cooling effectiveness. Another problem associated with using apertures to establish film cooling is that cooling air is dispensed from discrete points, rather than along a continuous line. The gaps between the apertures, and areas immediately downstream of those gaps, are exposed to less cooling air than are the apertures and the spaces immediately downstream of the apertures, and are therefore more susceptible to thermal degradation.
Another cause of ineffective cooling stems from the inability of some current designs to get cooling air where it is needed. Referring to FIG. 6, in a conventional airfoil the trailing edge cooling apertures typically extend between an upstream first cavity and to the pressure side exterior surface. The trailing edge cooling apertures generally include a meter portion and diffuser downstream of the meter portion. The diffuser has a surface profile that includes an upstream edge and a downstream edge. Under typical operating conditions: the static pressure (P1) at the upstream edge is greater than the static pressure (P2) at the exit of the meter portion; the static pressure (P3) at the entrance to the meter portion is equal to or less than the static pressure (P2) at the exit of the meter portion; and the static pressure (P3) at the entrance to the meter portion is equal to that within the cavity (P4). The relative static pressure values may be expressed as follows: P1 greater than P2, P2xe2x89xa7P3, and P3=P4. Note that these pressures reflect the static pressure of the flow, which may not equal the total pressure at any particular position. Total pressure is the sum of the dynamic pressure and the static pressure of the flow at any particular position. The dynamic pressure reflects the kinetic energy of the flow by considering the flow""s velocity at that particular position.
In those applications where the above pressure profile exists, cooling apertures (shown in phantom for explanation purposes) cannot be disposed between the first cavity and the outer surface of the airfoil because of the pressure difference across the apertures. Specifically, the static pressure P1 at the outer surface, which is greater than the static pressure P4 in the first cavity (i.e., P1 greater than P4), would cause undesirable hot gas inflow through the apertures. Cooling apertures upstream of the trailing edge must tap into a second cavity upstream of the first cavity that contains cooling air having a static pressure P5) greater than the static pressure at the trailing edge (P1; P5 greater than P1). For practical reasons, cooling apertures tapped into the second cavity are spaced a relatively long distance from the trailing edge cooling apertures. Cooling air exiting from those apertures is often ineffective at cooling the region upstream of the trailing edge cooling apertures located on the pressure side.
Hence, what is needed is a cooling apparatus and method that uses less cooling air and provides greater cooling effectiveness than conventional cooling schemes, one that helps create a uniform film of cooling air, and one that permits versatility in the positioning of cooling apertures.
It is, therefore, an object of the present invention to provide an apparatus and method for cooling a wall that provides convective cooling within the wall.
It is another object of the present invention to provide an apparatus and a method for initiating film cooling along a wall.
According to the present invention, a cooling circuit is provided disposed between a first wall portion and a second wall portion that includes one or more inlet apertures and one or more exit apertures. The inlet aperture(s) provides a cooling airflow path into the cooling circuit and the exit aperture(s) provides a cooling airflow path out of the cooling circuit. The cooling circuit includes a plurality of first pedestals extending between the first wall portion and the second wall portion. The first pedestals are arranged in one or more rows. According to one aspect of the present invention, adjacent first pedestals in any particular row are separated from one another by an intra-row distance, and adjacent first pedestals in adjacent rows are separated by an inter-row distance. The intra-row distance is greater than inter-row distance.
According to another aspect of the present invention, the passages formed between adjacent first pedestals in adjacent rows include a diffuser to diffuse cooling air flowing through the passage and a pair of throats to accelerate cooling air flow.
An advantage of the present cooling circuit is that it promotes uniformity in the film cooling layer aft of the cooling circuit. One aspect of the present cooling circuit that promotes film cooling development (which in turn leads to film layer uniformity) is the spacing of the pedestals. It is our experience that the inter-row and the intra-row pedestal spacing described herein below promotes lateral dispersion of cooling air within the cooling circuit better than any cooling arrangement of which we are aware. The increased lateral dispersion, in turn, produces a more uniform film cooling aft of the circuit.
Another aspect of the present cooling circuit that promotes uniformity in the film cooling layer aft of the cooling circuit is the compartmentalization provided by the cooling circuit. Each cooling circuit is an independent compartment designed to internally provide a plurality of incremental pressure drops between the inlet aperture(s) and the exit apertures. The incremental pressure drops increase the likelihood there will always be a positive flow of cooling air into the cooling circuit. The positive flow of cooling air through the circuit, in turn, positively affects the cooling circuit""s ability to create film cooling aft of the circuit.
The present invention""s ability to use a low pressure drop across the inlet aperture(s) provides another substantial benefit. A person of ordinary skill in the art will recognize that conventional casting cores used to create conventional cooling passages are notoriously difficult to handle and use because of their frailty. The frailty of a conventional casting core is particularly acute in the portion used to form the inlet aperture(s) because of the small diameter of the inlet aperture(s) (the small diameter is used to create a considerable pressure drop). The cooling circuit of the present invention allows for an inlet aperture diameter appreciably greater than that conventionally used without sacrificing cooling performance. We have found that the more robust casting core possible with the present invention may increase casting yields as much as 50%.
Some embodiments of the present invention include specialized exit apertures that promote uniformity in the film cooling layer aft of the cooling circuit. The aft most rows of pedestals include a plurality of mating second and third pedestals alternately disposed across the width of the cooling circuit. Cooling air flow encountering the second and third pedestals must travel first through an initial passage section between the heads of adjacent second and third pedestals, subsequently through a straight passage section, and finally into a diffuser passage section. The initial passage sections have a substantially constant cross-section that meters the cooling air as it enters the exit apertures. The initial passage sections follow the contour of the pedestal heads for a distance to minimize flow separation aft of the head of each second pedestal. Flow separation behind a blunt body pedestal can create undesirable cooling characteristics. The straight passage section has substantially the same cross-section as the initial section. Fluid flowing through the straight section, therefore, does not accelerate but rather settles the fluid flow prior to its entering the diffuser passage section with no appreciable pressure losses. Any entrance effects that may exist within the flow exiting the initial passage section are substantially diminished within the straight passage section prior to reaching the diffuser passage section. The straight passage section, therefore, performs a different function than the metering portion of a conventional diffused cooling aperture. The metering portion of a conventional diffused cooling hole is used to decrease the pressure of a fluid passing through the metering portion. The decrease in pressure across the metering portion is accompanied by an acceleration (i.e., a positive change in velocity) of the fluid passing therethrough. One of the consequences of the change in fluid velocity is the appearance of entrance effects within the boundary layer velocity profile. In our experience, fluid characterized by entrance effects that enters a diffuser does not diffuse as uniformly as does more settled flow. It is our further experience that settled flow entering the diffuser portion diffuses more readily, consequently promoting greater uniformity in the film cooling layer aft of the cooling circuit.
The embodiment of the present cooling circuit that includes a diffuser section in the passage between adjacent first pedestals provides an additional advantage in the form of enhanced convective cooling. Each passage between first pedestals includes a diffuser disposed between a pair of throats. Flow passing through the upstream throat will decelerate in the diffuser and subsequently accelerate passing through the downstream throat. Positioning the diffuser between the throats in this manner creates at least two regions of transient fluid velocity within each passage. The regions of transient fluid velocity are characterized by boundary layer entrance effects that have an average convective heat transfer coefficient higher than would be associated with fully developed fluid flow in a straight passage under similar circumstances. The higher heat transfer coefficient positively influences the heat transfer rate individually within the passage and collectively within the cooling circuit.
Another advantage of the present invention cooling circuit is the versatility it provides in terms of cooling aperture placement. As stated above, one of the hottest areas on an airfoil is immediately upstream of the trailing edge cooling apertures on the pressure side surface of the airfoil. The compartmentalized nature of the present cooling circuits, and the incremental pressure drops created therein permit the inclusion of additional cooling apertures within the cooling circuit. In the application of a cooling circuit disposed along the trailing edge of an airfoil, the additional apertures immediately upstream of the trailing edge exit enables the delivery of cooling air to that hottest point on the airfoil.
These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings.